Playback lag on half quality on Macbook Pro?

When i playback my sequences it plays a short piece without problems but after a while it start (sometimes) lagging.
It's 1080p footage and most of the time no effects added yet, so what on earth can be so difficult?
Ok its not rendered but it should be able to play normal on half resolution quality right?
I edit on one 4k external display and the laptop screen itself, but with or without it has almost the same performance.

Just installed the cuda driver and changed the render settings in Project settings.
Waaaoh what a difference! How stupid that i was thinking this lag was normal xd
Thanks for the replys! Now i am finally using the power of my GPU :)